Resolve "Fix LLM::CompletionWorker failure of summary too large issue"

What does this MR do and why?

Sometimes the LLM returns a large response in return when requesting a summary of the diff.

In sentry, we can see it happens infrequently, but does occur. We should handle the case where it does return more than the size of the database column and not save the summary in this case.

Summary examples:

A normal summary from MR as an example:

The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.

This is 400 words. If I multiply it to the limit, it would look like:

The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.The changes in this merge request introduce a new approach to handling approval rules when merging a merge request. This new approach involves creating a new column called "applicable_post_merge" which will be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules are applied and will make it easier to manage the approval process.be used to track whether or not a rule should be applied after the merge. This will allow for more flexibility in how approval rules

Related to #430222 (closed)